Ecological Equivalence: Niche Theory as a Testable Alternative to Neutral Theory - Recent Advances and Issues in Environmental Science - page 10

these outcomes over all frequencies. It is their good fit to steady-state patterns of
diversity and abundance even for communities subject to species turnover in ecologi-
cal drift that has argued powerfully for niche differences having a limited role in com-
munity structure. The Figure 3 simulations reveal these types of patterns to be equally
well represented by niche models, however, despite constituent individuals and spe-
cies achieving fitness equivalence only at dynamic equilibrium. Non-neutral dynamics
of a mature community express the community-wide average of fluctuations either
side of equilibrium. Outcomes regress to the equilibrium mean for a random assembly
of species undergoing stochastic extinctions of rare members, regulated by spatially
autocorrelated immigration, and replacement by initially rare invaders. The predicted
power of neutral theory can be taken as evidence for ecological equivalence at the co-
existence equilibrium of species with more or less different intrinsic attributes.
Modeling zero-sum ecological drift as an emergent property reveals a key dis-
tinguishing feature of truly neutral communities. Their intrinsically identical species
self-regulate to a lower total density as a result of inter-specifi c impacts equaling intra-
specifi c impacts. Any empirical test for competitive release is therefore also a test for
niche structure. For example, removing habitat is predicted to give a relative or abso-
lute advantage to species towards the fugitive end of a dominant-fugitive spectrum,
which may be picked up in correlated life-history traits for winners or losers under
habitat loss or degradation [23, 24]. In contrast, neutral dynamics lead to sudden bio-
diversity collapse at a system-wide extinction threshold of habitat [17]. The extinction
threshold of habitat for a resource-limited metapopulation is set by the fraction 1 / R
[30, 31]. The value of R is thus an important yardstick of resilience in conservation
planning. A neutral model fi tted to empirical zero-sum abundances will overestimate
their community-wide R , and hence overestimate community resilience, if α ij are over-
valued by setting all to unity. Likewise, a neutral model that sets all α ij = 0 ( i ≠ j ) will
underestimate R , and hence resilience, if the α ij are undervalued by setting all to zero.
Ecological equivalence is a much more permissive requirement for neutrality than
is currently acknowledged in theoretical developments on HNT. Co-existence equilib-
ria largely achieve the neutrality-defi ning mission, to eliminate all of the forces com-
peting for a place in explanations of pattern. It remains an open question whether they
do so best among species with most or least competitive release in each others' pres-
ence (e.g., Figure 1 versus Figure 2 respectively, and Figure 3 dominant-fugitive versus
Lotka-Volterra respectively; [7, 10, 32]). Models need to incorporate the ecologically
realistic dynamics of interspecifi c interactions simulated here in order to explore the
true nature of competitive release between extreme scenarios of niches that are all in-
trinsically identical (HNT [1]) and intrinsically unique [13], [16]. Simulations of nich-
es distributed along environmental gradients have found emerging groups of intrinsi-
cally similar species over evolutionary timescales [33]. For the spatially homogeneous
environments modeled here, competition-recruitment trade-offs will always sustain
species differences. In their absence, however, homogenous environments will tend to
favor fast-recruiting competitive dominants. This species type may eventually prevail,
with runaway selection checked by other forces such as predation, disease, mutation
accumulation, and environmental variability. These systems would merit further study
because many of their attributes could be those of intrinsically neutral dynamics.